Imaging optical systems relating to oblique image reading or image projection (hereinafter referred to simply as “oblique-incidence imaging optical systems”) are classified into oblique-incidence imaging optical systems of a decenter system and oblique-incidence image-forming optical systems of a tilt system.
FIG. 24 illustrates the basic principle of the oblique-incidence imaging optical system of the decenter system. In this oblique-incidence imaging optical system of the decenter system, an object plane 4 and image plane 2, which are conjugate planes, are basically parallel to each other, and the optical axis 3A of an image-forming optical system 30 is perpendicular to both the object plane 4 and the image plane 2. To realize an oblique-incidence imaging optical system, for example, an image detecting region 201 included in the image plane 2 is shifted below the optical axis 3A. Consequently, an image pick-up region 401 included in the object plane 4 is shifted upward as viewed in FIG. 24 and a oblique-incidence image-forming optical system can be realized without using any special optical system. The decenter system is advantageous in that any excessive distortion does not occur. However, the same system is disadvantageous in that the image circle of the image-forming optical system 30 must be large to displace the optical axis 3A, the correction of aberration is difficult and the imaging optical system 30 is large.
FIG. 25 illustrates the basic principle of the tilt system. The tilt system differs greatly from the decenter system in that the optical axis 3A of an imaging optical system 30 is oblique to an object plane 4, and an image plane 2 is oblique to the optical axis 3A. Respective prolongations of the image plane 2, the object plane 4 and the principal plane of the imaging optical system 30 intersect on a line A of intersection to meet Scheimpflug's principle, i.e., an imaging condition for the tilt system. The tilt system is advantageous in that the imaging optical system 30 is not excessively large and resolving power is comparatively high. The same system is disadvantageous in that a new distortion occurs. FIG. 26 shows a typical example of such a distortion, which can be readily understood from the examination of imaging magnification illustrated in FIG. 25.
Oblique-incidence imaging optical systems are classified into those of the decenter system, those of the tilt system and those of a composite system having the characteristics of both the decenter and the tilt system. The image-forming optical system must meet predetermined conditions about particulars including resolving power and distortion required of the optical system. Various devices have been proposed to solve problems in those systems and efforts have been made to provide optical systems answering purposes. Some examples of prior art optical system will be described.
FIGS. 27(a) and 27(b) show a projection lens for a projector disclosed in JP-A No. Hei 05-273460 in a sectional view. A projection lens 30 consisting of refracting optical elements, and an image-forming device 2 are moved perpendicularly to the optical axis 3A of the projection lens 30 relative to each other to realize an oblique-incidence imaging optical system. To avoid moving a condenser lens 301 disposed near the image-forming device 2, the optical axis of the projection lens 30 is tilted when moving the projection lens 30. Therefore, it is considered that this oblique-incidence imaging optical system is basically of the decenter system and uses tilting for the degree of freedom of correction. This optical system achieves image projection in a maximum field angle 2ω of about 51°.
FIG. 28 shows a projector disclosed in U.S. Pat. No. 5,871,266 to the applicant of the present invention patent application in a sectional view. The projector includes, as essential components, an illuminating system 1 including a light source, an image-forming device 2 including a liquid-crystal display or the like, and an imaging system 3. The illuminating system 1 and the imaging system 3 are optimized to realize an oblique-incidence imaging optical system. In a concrete example, the imaging system 3 comprises only a small number of reflecting mirrors. A light beam emitted by the illuminating unit 1 is decomposed into light beams of three primary colors by dichroic mirrors 2a and 2b to illuminate three reflecting image-forming devices 2g, 2h and 2i. Light beams reflected by the reflecting image-forming devices 2g, 2h and 2i are combined by the dichroic mirrors 2a and 2b, and the light beams travel toward the imaging system 3. The imaging system 3 has three reflecting mirrors 3a, 3b and 3d. The light beams from the reflecting image-forming devices 2g, 2h and 2i are reflected by the reflecting mirrors 3a, 3b and 3d to form an image on a screen 4, not shown. In this specification, the significance of the oblique-incidence imaging optical system included in the projector is discussed minutely. An example applied to a thin rear projection display capable of achieving image projection in a maximum field angle 2ω exceeding 100°. This rear projection display is, basically, of the decenter system.
Although such an epoch-making projector can be realized, the system disclosed in U.S. Pat. No. 5,871,266 has some disadvantages. The reflecting mirrors of the imaging system, as compared with refracting optical elements, must be formed in a high surface accuracy, which will be readily understood from the imagination of the state of reflection of imaging light by the reflecting mirrors. For example, suppose that a light beam emitted by the image-forming device to be focused on a point on a screen forms a spot in a region of a reflecting surface. If the region has a form error of λ/4, where λ is, for example, 0.55 μm, a wave aberration of about λ/2 is produced. This wave aberration causes a nonnegligible reduction in the resolving power of the imaging optical system. Thus, the accuracy of the catoptric system is affected significantly by waviness errors in the reflecting surface.
Another disadvantage of the system is the incidence angle of a light beam from the image-forming device. As stated in claims, a divergent light beam diverging at a divergence angle of 8° or below is used to realize a simple oblique-incidence imaging optical system. In this patented invention, all the systems including an illuminating system are optimized to enhance the efficiency of light. However, the system has only a narrow application field because of various restrictions on the size of an available light source, and the size and costs of the device.
An invention disclosed in JP-A No. Hei 10-206791 relates to a projector of the decenter system. As shown in FIG. 29, this invention includes an imaging system 30 employing decentered optical elements and free-form surfaces to increase the degree of freedom of design, and realizes a projection system having a maximum field angle 2ω exceeding 68°. The imaging system 30 is used as an imaging system for oblique projection as shown in FIG. 30. In FIG. 30, two conjugate planes 2 and 4 are substantially parallel. Although such decentered optical elements are employed, the field angle is not increased and, on the other hand, difficulty in fabricating and assembling parts is enhanced.
Some known optical systems of the decenter system have been described by way of example. Known optical systems of the tilt system will be described hereinafter.
FIG. 31 illustrates an invention disclosed in U.S. Pat. No. 5,274,406 relating to a projector particularly for a rear-projection display. This projector includes a symmetric projection lens 30 consisting of refracting optical elements shown in FIG. 32, and a free-form surface mirror 301 having minute setbacks similar to those of a Fresnel lens as shown in FIG. 33(b) and disposed near an image plane. The optical axis of the projection lens 30 is tilted relative to a screen 4 and an image-forming device 2 to form the rear-projection projector in a small depth. A distortion resulting from the tilting of the optical axis is corrected by a free-form surface mirror shown in FIG. 33(a), and problems relating to the mismatch of imaging conditions resulting from the use of such mirrors are dealt with by forming minute setbacks similar to those of a Fresnel lens in their surfaces.
Thus, a rear-projection display provided with a 36 in. screen having a diagonal length 36 inch is formed in a thickness of 28 cm, which is smaller than a target thickness, because it is said that a normal number of the target thickness in inch equal to a normal number of the diagonal of the screen in centimeter. Although the rear-projection display can be thus formed in a small thickness, the distance D1 between the projection lens 30 and the free-form surface mirror 301 along an optional light beam is greater than the distance D2 between the free-form surface mirror 301 and the screen along the same light beam, and hence the free-form surface mirror 301 is necessarily large. Therefore, the fabrication of the free-form surface mirror having the setbacks similar to those of a Fresnel lens is very difficult. The free-form surface mirror 301 having a surface resembling that of a Fresnel lens and employed to prevent the reduction of resolving power has the finite stepped construction and the steps reduces resolving power.
FIG. 34 shows other oblique-incidence imaging optical systems of the tilt system disclosed in JP-A Nos. Hei 06-265814 and Hei 07-151994. These oblique-incidence imaging optical systems employs a plurality of optical systems of the tilt system to correct a distortion. For example, an oblique-incidence imaging optical system shown in FIG. 35 has two optical systems of the tilt system. A light beam emitted by an image-forming device included in a plane 2 is focused by a first imaging system 3 to form an intermediate image on a plane 4. A second imaging system 3′ forms an image of the intermediate image on a screen 4′. Theoretically, it is possible to correct the distortion and to prevent the reduction of resolving power by properly determining the angle, magnification, focal length and such of each optical system so as to meet predetermined conditions. Since the respective optical axes of the imaging systems 3 and 3′ of this oblique-incidence imaging optical system intersect the plane 4 on which the intermediate image is formed at predetermined angles. Therefore the light beam must travel from the optical system 3 to the optical system 3′ without being shaded. Generally, a pupil-coupling device, such as a decentered Fresnel lens as shown in FIG. 36, is disposed at a position where the intermediate image is formed to enable the light beam to travel from the optical system 3 to the optical system 3′ without being shaded. For example, if the image-forming device has minimum pixel construction like a liquid crystal panel, a moiré pattern is formed by interference with the periodic construction of the Fresnel lens. This problem is solved by displacing the pupil-coupling device from a position corresponding to the intermediate image.
This oblique-incidence imaging optical system is disadvantageous in that the respective optical axes of the optical systems 3 and 3′ are inclined at a large angle to the intermediate image 4 or the image-forming device 2 and it is often difficult to meet mechanical requirements. Detailed description of the problem will be omitted herein. The pupil-coupling device shown in FIG. 36 is one of problems difficult to solve.
FIG. 37 shows an imaging optical system disclosed in JP-A No. Hei 07-13157. A collimated light beam emitted by a light source 1a falls on an image-forming device 2. A first paraboloidal reflector 3a concentrates the light beam reflected by the image-forming device 2 on the pupil of a projection lens 3b. The light beam traveled through the projection lens 3b is reflected by a second paraboloidal reflector 3c to form an enlarged image on a screen 4. Basically, this imaging optical system is of the tilt system. The first paraboloidal reflector 3a is used for coupling the projection lens 3b and an illuminating light beam, and the second paraboloidal reflector 3c is used to make the light beam fall on the screen 4 in a direction at a fixed angle to the screen 4 to form a thin rear projection display. Although it is unknown whether the invention is practically realizable because concrete examples are not mentioned in the specification, it is considered that this imaging optical system is unable to meet practical optical requirements.
FIG. 38 shows an imaging optical system of the tilt system disclosed in JP-A No. Hei 09-179064. This imaging optical system, similarly to those disclosed in U.S. Pat. No. 5,871,266 and JP-A No. Hei 07-13157, includes, in combination, an imaging system 30 consisting of refracting optical elements, and a concave reflector 31. A light beam emitted by an image-forming device 2 travels through the optical system 30 consisting of refracting optical elements 3a to 3g shown in FIG. 39, is reflected by the concave reflector 31 and falls obliquely on a screen 4. This imaging optical system utilizes the characteristics of an afocal system for correcting a distortion produced by the tilt system.
When an afocal system is constituted of two optical elements 30 and 31 such that the distance between the optical elements 30 and 31 is equal to the sum of the respective focal lengths of the optical elements 30 and 31 as shown in FIG. 40, the magnification of the afocal system remains constant regardless of an object distance. A distortion can be corrected by constituting such an optical system of a refracting optical element 30 having a positive focal length and a concave reflector 31 having a positive focal length, and by making a light beam fall on a screen 4 at a fixed angle to the screen 4.
In an embodiment of this known optical system, a light beam falls on the screen 4 in an object plane at a large angle of, for example, 70° to a normal to the screen 4. A decentered optical element and a free-form surface are employed to secure a degree of freedom to reduce the distortion further and to improve resolving power. This system is disadvantageous in that the afocal system is constituted of the two optical elements, and the distance between the optical elements 30 and 31 must be unavoidably increased to form an enlarging system; that is, when the distance between the projection lens 30 and the concave reflector 31 along a light beam is D1 and the distance between the concave reflector 31 and the screen 4 along a light beam is D2, D1>D2 for most part of the light beam and hence the concave reflector 31 must be necessarily large, which causes problems in the mass production of the concave reflector 31.
The foregoing examples are techniques mainly relating to projectors. Let us examine some examples of head-mounted displays (HMDs) as other possible use of oblique-incidence imaging optical systems.
Important matters to be taken into account in designing a head-mounted display are: wideness of angle of field (large enlarged image), smallness of dimensions, and lightness.
Regarding angle of field, a necessary size of an image-forming device is substantially dependent on the connection with an angle including an image-forming device when a necessary angle of field is specified, because the size of a pupil is substantially fixed. FIG. 41 shows a standard HMD. A relay optical system 30 focuses a light beam from an image-forming device 2 to form an intermediate image 4, and a concave mirror 31 enlarges the intermediate image to provide an enlarged image. The enlarged image is observed with an eye at a position 303. The concave mirror 31 concentrates principal light beams on the pupil. Basically, this HMD is a coaxial system, which is an optical system easy to design. However, since the eye must be spaced from the concave mirror 31, and the relay optical system 30 needs a space for installation, the HMD is inevitably considerably large.
FIG. 42 shows the optical system of a HMD disclosed in JP-A No. Hei 05-303055. An imaging optical system including a relay optical system 30 focuses a light beam from an image-forming device 2 to form an image, and a concave mirror 31 enlarges the image to provide an enlarged image. The enlarged image is observed with an eye at a position 301. Basically, this HMD is the same in configuration as the aforesaid HMD. This HMD omits a beam splitter and employs a decentered system to form the HMD in s small thickness. Thus, this HMD is an oblique-incidence imaging optical system of the tilt system.
A HMD disclosed in JP-A NO. 07-191274 is obtained by introducing improvements in the HMD disclosed in JP-A No. Hei 05-303055. As shown in FIGS. 43 and 44, one concave mirror is replaced with a plurality of convex mirrors and concave mirrors to improve the correction of aberration. Addition of the convex mirror increases the degree of freedom of high aberration correction and widens the width of design. The reflector closest to the eye is a concave mirror. A HMD in an embodiment has a relay optical system 30 consisting of reflectors and consisted entirely of a catoptric system. This HMD is analogous with the projection optical system of the projector disclosed in U.S. Pat. No. 5,871,266 in constituting the optical system only of reflectors.
FIG. 45 shows a HMD disclosed in JP-A No. Hei 10-239631. This HMD is formed in compact construction by folding the arrangement of the reflectors of the HMD disclosed in JP-A No. Hei 07-191274 in a space. Although the HMD is small, aberration is corrected effectively by means of two refracting surfaces 301 and 304 and two reflecting surfaces 302 and 303. To secure degree of freedom, optical surfaces are free-form surfaces. This system is epoch-making for applications in which two image-forming device s for both eyes can be used and a comparatively large f number is permitted.
Although examples of optical systems in the two fields of application of the oblique-incidence imaging optical system have been described, the oblique-incidence imaging optical system is applicable to various uses, and the field of practical application of the same to various products has been progressively widening. For example, there have been proposed new oblique-incidence imaging optical systems meeting current requirements, such as the oblique-incidence imaging optical system proposed in JP-A No. Hei 10-239631, for the field of HMDs. However, those oblique-incidence imaging optical systems are unsatisfactory in meeting future requirements for wide angle of field and picture quality. The degree of freedom may be increased by increasing reflecting surfaces as mentioned above in connection with JP-A No. Hei 07-191274. However, increase in the number of reflecting surfaces requires forming the reflecting surfaces in high accuracies and increases the cost. Thus further technical research and development in this field is desired.
When the oblique-incidence imaging optical system is applied to a projector or an image pickup system, further improvement of the ability of the oblique-incidence imaging optical system is required, because a projector or an image pickup system requires abilities severer that those required of systems for visual observation. Image-forming devices, such as liquid crystal displays, and image pickup devices, such as CCDs, have been progressively miniaturized and pixels have been reduced to sizes on the order of micrometers. Consequently, optical systems having high resolving power and capable of preventing reduction in light intensity are needed. On the other hand, the miniaturization of devices is advantageous conditions for the miniaturization of optical systems. If an image can be projected at a half field angle exceeding 70° as mentioned in U.S. Pat. No. 5,871,266, a display can be formed in a thickness equal to ⅓ of the thickness of the conventional display, and can be applied to various input/output devices including a videophone system disclosed in JP-A No. Hei 06-133311 as shown in FIG. 46, projectors, thin image readers capable of reading an image at a stroke, such as image scanner, stereoscopic image readers and cameras.
Thus, a technical subject of the present invention is to increase means for realizing an oblique-incidence imaging optical system as much as possible. Unfortunately, the conventional optical systems have some problems in brightness, resolving power, size, productivity and/or costs, and only few conventional oblique-incidence imaging optical systems are widely applicable to many uses.
It is an object of the present invention provides new means for realizing an oblique-incidence image-forming optical system and to apply the same to various uses.
Another object of the present invention is to provide means for realizing a bright oblique-incidence image-forming optical system capable of projecting an image at a half field angle exceeding 70° which could not be achieved by prior art and of controlling distortion.